3-aminomethyl-3,5,5-trimethylcyclohexyl amine, designated in the following as isophorone diamine, is used as a starting material for the preparation of isophorone diisocycanate, as an amine component for making polyamides and as a hardener for epoxide resins.
Isophorone diamine (IPDA) has been obtained in the past by the reductive amination of 1,3,3-trimethyl-5-oxocyclohexane carbonitrile, designated herein as "isophorone nitrile" (IPN), in the presence of ammonia and conventional hydrogenation catalysts. The isophorone nitrile used as a starting material can be obtained by means of the addition of hydrogen cyanide to isophorone - see German patent application P 39 42 371.9 (cf. Canadian Patent 2,032,667).
German patent DE 12 29 078, discloses using ammonia and IPN in a molar ratio of 10-30:1 in order to obtain IPDA. However, a rather large amount of byproducts are produced in addition to the desired IPDA, such as in particular 3-aminomethyl-3,5,5-trimethylcyclohexanol (also known as isophorone amino alcohol (IPAA)), 1,3,3-trimethyl-6-azabicyclo-(3,2,1) octane and dihydroisophoryl amine. A yield of up to 81.4% IPDA is disclosed by way of example; however, there is no purity data. The cited yield proved to be non-reproducible, as has been determined by various parties.
In an attempt to obtain a high yield of IPDA and to minimize the undesired accumulation of IPAA, the method of German patent DE 12 29 078 was modified in accordance with the disclosure of published German patent application DE-OS 30 11 656 in such a manner that IPN was converted free of catalyst in a first stage with excess ammonia into 1,3,3-trimethyl-5-imino-cyclohexane carbonitrile and the latter was hydrogenated in a second stage to IPDA. A considerable excess of ammonia had to be used in the second stage. This method of operation requires an expensive pressure distillation for recovery and recycling of the ammonia. According to the example given, a reaction yield of only 83.7% was achieved in the method of DE-OS 30 11 656 in spite of using a ratio of approximately 5 kg ammonia per 1 kg IPN; no data was supplied about the yield of isolated IPDA and its purity.
Published German patent application DE-OS 30 21 955 (cf. U.S. Pat. No. 4,429,157) makes it clear that there was a need to improve the methods of the previously cited documents. According to reference Example 1 of DE-OS 30 21 955, an IPDA yield of only 48% is achieved, in spite of an IPN/NH.sub.3 volumetric ratio of 1 to 10, in a method carried out by analogy with German patent DE 12 29 078. An unspecified "commercially available catalyst" was used in this instance. According to reference Examples 2 and 3 of DE-OS 30 21 955, carried out by analogy with published German patent application DE-OS 30 11 646, it was possible to obtain a yield of approximately 70% and 90%; however, a long reaction time was required for the first stage and an IPN/NH.sub.3 volumetric ration of 1 to 10 in the second stage was necessary for this. Thus, in addition to the disadvantage of the high ammonia excess, there is also an economically significant reduction of the space-time yield.
Published German Patent Application DE-OS 30 21 955 discloses that it is possible to reduce the long reaction time for the first stage--imine formation--in the method of DE-OS 30 11 656 by using an imine-formation catalyst. However, this increased the cost of the reaction, in addition, it still was necessary to use a very high ammonia/IPN volumetric ratio.
In the methods of DE-OS 30 11 656 and DE-OS 30 21 955, carrier-free or carrier-bound Co-, Ni-, Fe- and noble metal catalysts, among others also Raney nickel and Raney cobalt, are designated as suitable. A possible co-use of catalytic amounts of acids and ammonium salts is referred to in DE-OS 30 11 656 but without disclosing details about the type of material to use.
Published Japanese Patent Application JP-A-62-123154 discloses a method for the reductive amination of IPN for the preparation of IPDA in which an attempt is made to reduce the required excess of ammonia and to eliminate the preliminary reduction of the carrier-bound catalyst. According to this method, it should be possible to obtain IPDA in high yield--yields between 83 and 89% in the reaction mixture are indicated in the examples--if a 1 to 20-fold, preferably 5 to 10-fold molar amount of ammonia is used, relative to IPN, as well as Raney cobalt as catalyst, a pressure of 50 to 150 bars and a temperature of 50.degree. to 150.degree. C.
By reworking the method of JP-A 62-123154, it has been determined that, in spite of increasing of the excess of ammonia over the value indicated in the Japanese document, almost no absorption of hydrogen took place and Raney cobalt alone is therefore practically ineffective.